Cardiomyocytes in the adult mammal exhibit little if any capacity to undergo cell division. Consequently cardiomyocyte loss due to injury or disease is irreversible. Transplantation of donor cells (either cardiomyocytes or stem cells with cardiomyogenic potential) has emerged as a novel way to treat damaged hearts. Recent studies have demonstrated that adult-derived stem cells display a greater degree of plasticity than previously anticipated, and several studies have presented data suggestive of cardiomyogenic differentiation from adult bone marrow-derived stem cells. The experiments proposed in this application will assess the cardiomyogenic potential of three independent bone marrow stem cells, namely Side Population (SP) stem cells, lin-/c-kitHI stem cells, and Multipotent Adult Progenitor Cells (MAPCs). In Aim 1 will determine the capacity of the various bone marrow stem cells to undergo cardiomyogenic differentiation in vitro in response to known growth factors and under co-culture conditions with cardiomyogenic cells. In Aim 2 the undifferentiated stem cells will be transplanted into developing embryos or into normal and injured adult hearts to determine their ability to respond to cardiomyogenic cues during development and following myocardial injury. Aim 3 will determine if cardiomyocytes produced by in vitro differentiation of the bone marrow stem cells can be used for transplantation. Finally, Aims 4 and 5 will determine if genetic interventions which permit the purification and amplification of ES-derived cardiomyocytes can be applied to bone marrow stem cell-derived cardiomyocytes. These experiments will provide the first comparative analyses of the cardiomyogenic potential of multiple bone marrow stem cells in vivo and in vitro, and will also establish the genetic tractability and malleability of the cells. In this regard, the application is responsive to four of the seven focus areas suggested in the RFA. The information obtained from these studies will establish the feasibility and potential utility of bone marrow stem cells as a source of transplant cardiomyocytes for the treatment of cardiovascular disease.